In a series of tweets, the president said, "After consultation with my Generals and military experts, please be advised that the United States Government will not accept or allow ... Transgender individuals to serve in any capacity in the U.S. Military."

Acting Secretary of the Army Robert Speer said he couldn't comment on where the White House is "going with the policy" when asked for his views at an Association of the United States Army event.

"I don't have my cell phone, so I'm not checking Twitter," Speer said with a chuckle. "To be tongue and cheek, Twitter is not a policy.

"I do know that we were encouraging the current SecDef to review the accessions policy on transgender, which he was doing and that review and analysis was not done. And that's the latest guidance, so I'm not going to get in between where the president or the SecDef is right now on where they are going on a future policy."

Speer added, "I know the current policy and the current one particularly on accessions we were holding off until we studied it a little further."

His comments are the most expansive yet of any of the civilian leaders within the services on the Trump's announcement.

The Pentagon didn't immediately release additional details on the president's announcement and referred all questions about the matter to the White House.

Earlier this month, a spokeswoman for Defense Secretary Jim Mattis said he was giving the service chiefs another six months to review whether allowing transgendered individuals to serve would hurt the "readiness or lethality" of the force, The Associated Press reported.

The Army and Air Force had requested a two-year wait, while the Navy had requested a one-year wait in part to accommodate a request from the Marine Corps, according to the AP.

The delay ordered by Mattis did not affect transgender troops who were already serving openly in the military, according to Pentagon spokeswoman Dana White. It wasn't immediately clear what will happen to these individuals.

By his action, Trump "has put a target on the backs of the more than 15,000 transgender troops proudly serving in our military" who could be subject to discharge, said Chad Griffin, president of the Human Rights Campaign advocacy group for the LGBTQ community (Lesbian, Gay, Bisexual, Transgender, Queer). HRC also estimates that there are about 134,000 transgender veterans.

Some 250 people are transitioning genders or who have been approved to change gender within the Pentagon's personnel system, the AP and other news organizations have reported.

There are between 1,320 and 6,630 transgendered troops currently serving on active duty, which amounts to between 0.1 percent and 0.5 percent of the 1.3 million-member active component, and between 830 and 4,160 in the Selected Reserve, according to a 2016 study by Agnes Gereben Schaefer, a senior political scientist at the nonprofit.

The Pentagon had no immediate guidance for transgender individuals who have committed to enlist or are currently serving.

"We will continue to work closely with the White House to address the new guidance provided by the commander-in-chief on transgender individuals serving in the military," Pentagon spokesman Navy Capt. Jeff Davis said. "We will provide revised guidance to the department in the near future."

In a statement, the American Civil Liberties Union said it's looking at legal options to challenge the Trump ban and asked transgender troops in the ranks to contact it.

"This is an outrageous and desperate action," said Joshua Block, a senior staff attorney with the ACLU's LGBT & HIV Project.

Others supported the Trump's announcement.

Tony Perkins, a Marine veteran and president of the Family Research Council, praised the president's decision.

"I applaud President Trump for keeping his promise to return to military priorities -- and not continue the social experimentation of the Obama era that has crippled our nation's military," Perkins said.

Former Defense Secretary Ashton Carter, who last summer allowed transgender troops to serve openly and directed the military to develop plans on recruitment, said it's Trump who is engaged in social experimentation.

"To choose service members on other grounds than military qualifications is social policy and has no place in our military," Carter said in a statement.

While Republican lawmakers, consumed by the battle in Congress on health care, didn't immediately comment on the Trump ban, some did.

Sen. Lindsey Graham, a Republican from South Carolina, echoed the criticism of Sen. John McCain, a Republican from Arizona and chairman of the Senate Armed Services Committee, that Trump should not be making policy in Tweets.

Graham called for a Senate hearing in which military leaders could testify on the fallout from the Trump ban and whether it would result in the discharge of transgender individuals now serving.

"We need to have a hearing, not a Tweet," Graham said. "Let the military tell us about the policy change -- what it is, does it affect the people currently serving, and what is the recommendation."